1. Field of the Invention
This invention relates to lasers and, in particular, to a laser imaging system for use in analyzing defects on semiconductor wafers.
2. Related Art
Semiconductor chip manufacturers have increasingly sought to improve yields in their production processes. Key to this effort is the reduction of particulate contamination during wafer processing. As the line widths of features on the chip have shrunk from 10 microns several years ago to one micron and below today (with line widths approaching 0.3 micron or less expected in the next few years), the ability to detect and control smaller and smaller particles to achieve higher degrees of cleanliness has become paramount. Additionally, production of acceptable chips requires accurate performance of each of the process steps carried out on the wafer. The value of product on each wafer has also increased dramatically, due to the increasing complexity of semiconductor devices (many more layers and process steps) and the development of larger wafers (up to 200 mm diameter), further accentuating the need for defect detection and control.
Instrument suppliers have addressed a portion of this problem by developing defect detecting systems which scan wafers (wafer scanners) during production for anomalous optical sites that are characteristic of particulate contamination (but may represent other flaws as well). Defects can be either a pit or a bump in the surface of the wafer.
In one type of wafer scanner, in which a laser beam is focussed on and scanned over the surface of the chip (laser scanning system), anomalous optical sites are identified by comparing the light scatter from locations on known good chips to the light scatter from the corresponding locations on the chips being tested. If the two light scatters are different, than an anomalous optical site has been detected. Wafer scanners of this type are made by Tencor Instruments of Mountain View, Calif. as Model 7500, and by Inspex of Billerica, Mass. as Model TPC 8500.
In another type of wafer scanner, a video picture is taken with a conventional video camera of the surface of a known good chip and compared to a corresponding video picture taken of a chip to be tested. Typically, these video systems use white light imaging. The video pictures are analyzed by comparing them on a pixel by pixel basis, i.e., numerical data representing the video image at each pixel is compared and, if the difference falls outside of a pre-established acceptable difference, an anomalous optical site is identified. KLA of San Jose, Calif. makes a wafer scanner of this type as, for example, Model 2131.
The video systems generally cost about three times as much as the laser scanning systems, i.e., the laser scanning systems typically cost approximately $350,000 while the video systems typically cost approximately $1,000,000. However, while the laser scanning systems are more effective in detecting bumps than in detecting pits, the video systems work well in detecting either bumps or pits, and can also sense subsurface defects.
As these wafer scanners were developed, the need to identify positively the nature, e.g., type of material, type of defect (defects are classified broadly as particulate or process flow defects; there are many sub-types within each of these classifications), and the precise location and size of the defects was not appreciated. This information is important for several reasons. Identification of the nature of the defect can be used to determine the origin of the defect. The number, location and size of the defects can be used to calculate the density of defects in general, and, along with identification of the nature of the defects, the density of particular types of defects. This information can then be used to more closely monitor and/or to modify process steps in the chip production process.
As the need for more precise defect analysis has become apparent, semiconductor manufacturers' demand for the ability to "revisit" defects (or a subset of them) found by the above-described wafer scanners, for purposes of positive identification of the nature, location and size of the defects, has led to the hasty design and production of review stations based on laboratory microscopes with precision wafer handling stages that allow an operator to close in on and evaluate the previously detected defects. Revisiting of the defects by the review stations is done off-line from the defect detection process so as not to limit the throughput of the wafer scanners. Little engineering was done in the design of these review stations: in particular with respect to the optics and cleanliness (e.g., the review stations typically use off-the-shelf, visible light, research-style microscopes).
As noted above, the decreasing line widths of features on current and future semiconductor chips increase the importance of detection of contaminants and other defects having a diameter, width, or other characteristic dimension on the order of 0.1 to 0.3 microns. The visible light, off-the-shelf microscopes currently being used in defect review stations lack sufficient resolution to resolve defects of such small size, or to resolve this size structure on larger defects to aid in identification. Visible light scanning microscopes (both white light and laser-based) that are built by modifying off-the-shelf microscopes can improve the resolution significantly, but they are currently in limited use, mostly as part of complex and expensive research setups. Additionally, the use of conventional microscopes increases the risk of contamination of the semiconductor chips during the review process, since a (relatively dirty) human is in close proximity to the wafer surface and because the presence of the microscope causes turbulent flow near the wafer which tends to pull in nearby contaminants to the wafer.
Consequently, the semiconductor processing industry has attempted to use scanning electron microscopes (SEMs) that will provide increased resolution and perform energy dispersive (EDX) analysis. In EDX analysis, X-rays are directed toward the surface of the semiconductor chip. By measuring the wavelength spectrum of the reflected light, information can be gleaned regarding the types of material present on the wafer surface. Unfortunately, EDX analysis requires high voltage (up to approximately 40,000 volts) SEMs; bombardment of the wafer surface with electrons from high voltage SEMs causes damage to the wafer, rendering the wafers unusable for further processing. Recently, low voltage SEMs (100-1000 volts) have seen limited use in wafer fabs for "critical dimension" measurements of line widths, but low voltage SEMs are too slow to use except on a sample basis, and, in addition, provide no analytical (i.e., EDX) capability. Further, in both high and low voltage SEMs, the time to load samples into the SEM and pump down the load-lock chamber containing the SEM is relatively long, undesirably slowing down processing of the wafers. As a result, defect revisiting with SEMs is usually done off-line in a quality control or analysis laboratory.
In an attempt to overcome the limitations of SEMs, some major semiconductor producers have begun to use systems which include both low and high voltage SEMs. However, such systems are expensive, selling in the $1,000,000 to $1,500,000 range.